1. Field of the Invention
The present invention relates to an optical device and an optical head using the same. More specifically, the present invention relates to an optical device having a very high throughput and resolution using surface-plasmon-enhanced optical transmission through a sub-wavelength hole in conductive films and a head for an optical recording medium using the same.
2. Description of the Related Art
An optical recording disc such as a CD (compact disc) and DVD (digital video disc) is an increasingly attractive data recording medium because of its high data recording density, compact design, portability, robustness and the lower costs of both the medium and a write device. Irrespective of a relatively high data density provided by the optical disc, a higher recording density is thought to be desirable. A smaller spot provides a higher data recording density on the optical recording medium and a higher data read speed to a given rotating speed of the medium. The recording density is limited by the diffraction limit of an optical spot on the medium. When using a lens or other far field focusing devices to focus the focal point of an optical beam onto the recording medium, the diameter size of a “spot” of the focused light at the focal point is limited to λ/2 (λ is the wavelength of a light.) by diffraction. This is known as the diffraction limit.
To increase the recording density above the current value, the size of the optical beam writing or reading data must be reduced. Such optical disc generally shows significant disadvantages when the read speed (the speed of data which can be read from the optical disc) is relatively low.
As a technique for overcoming the diffraction limit, a near field optical technique using the feature of a low phase speed of an evanescent wave receives attention. The evanescent wave having a short wavelength passing through a sub-wavelength hole holed in a metal film is used for optical write/read.
A flat metal film thicker than its own optical skin depth (the depth in which an electromagnetic field of an incident light is transmitted into a material until a field intensity is lowered to 1/e2, typically, 20 to 30 nm to a metal) is nontransparent to a light having a frequency lower than a bulk plasma frequency ωp. The balk plasma frequency ωp is given by ωp=(4πne2)/m* where n is an electron density, e is an electric charge, and m* is an effective mass. The transmission efficiency of a light passing through a single hole in such metal film depends on the diameter of the hole. When the hole diameter is smaller than the wavelength of a light passing through the hole, the transmissivity is in proportion to (d/λ)4.
When using such a small hole, the transmission through the hole in a prior art optical device using a near field, such as a tapered front edge of an optical fiber, is susceptible to severe attenuation. The signal-to-noise ratio is too low for reading and a sufficient optical intensity needed for writing cannot be obtained. A practical optical data read/write head using a near field optical system has not been obtained.
There has recently been disclosed an optical transmission technique which significantly enhances the transmissivity of a light passing through hole arrays using a metal film having light sub-wavelength hole arrays (see T. W. Ebbesen et al., “Extraordinary optical transmission through sub-wavelength hole arrays”, Nature, vol. 391, pp. 667–669, (Feb. 12, 1998), U.S. Pat. No. 5,973,316 T. W. Ebbesen et al., filed on Nov. 26, 1997 and provisional U.S. Patent No. 60/051,904, T. W. Ebbesen et al., filed on Jul. 8, 1997).
The transmissivity of a light passing through hole arrays is enhanced when a light incident upon a metal film resonantly interacts with a surface-plasmon mode. The surface plasmon mode (hereinafter called merely plasmon) is an excitation state of collective electrons which exist at the interface between a metal and a dielectric medium adjacent to it. The resonant interaction of the surface plasmon mode and the incident beam is occurred in the structure in which hole arrays are arranged.
In the prior art optical device with hole arrays, the hole arrays are characterized only as a hole transmitting a light. In an application such as write/read of an optical disc, it is desirable that the higher order transmission as shown in the hole arrays be realized by a single hole or a small set of holes. It is also desirable that the transmission of the hole arrays be enhanced. The sub-wavelength hole device showing a sufficient transmission efficiency has not been realized. In the high efficiency optical transmission using the surface plasmon shown by Ebbesen et al., a sufficient optical transmission efficiency has not been obtained yet in the prior art manufacturing method.
As described above, the read/write head for an optical data recording medium of an optical device using a near field optical system is not practical. To solve this, use of a transmitted light enhanced by the plasmon effect is proposed. But, in this case, it is a problem that the efficiency of the transmission light to the incident light is not sufficient.